# std::ranges::replace_copy, std::ranges::replace_copy_if, std::ranges::replace_copy_result, std::ranges::replace_copy_if_result

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< cpp‎ | algorithm‎ | ranges

Defined in header `<algorithm>` |  |
Call signature |  |
| (1) |
template< std::input_iterator I, std::sentinel_for<I> S, class T1, class T2, std::output_iterator<const T2&> O, class Proj = std::identity > requires std::indirectly_copyable<I, O> && std::indirect_binary_predicate <ranges::equal_to, std::projected<I, Proj>, const T1*> constexpr replace_copy_result<I, O> replace_copy( I first, S last, O result, const T1& old_value, const T2& new_value, Proj proj = {} ); |  | (since C++20) (until C++26)
template< std::input_iterator I, std::sentinel_for<I> S, class O, class Proj = std::identity, class T1 = std::projected_value_t<I, Proj>, class T2 = std::iter_value_t<O> > requires std::indirectly_copyable<I, O> && std::indirect_binary_predicate <ranges::equal_to, std::projected<I, Proj>, const T1*> && std::output_iterator<O, const T2&> constexpr replace_copy_result<I, O> replace_copy( I first, S last, O result, const T1& old_value, const T2& new_value, Proj proj = {} ); |  | (since C++26)
| (2) |
template< ranges::input_range R, class T1, class T2, std::output_iterator<const T2&> O, class Proj = std::identity > requires std::indirectly_copyable<ranges::iterator_t<R>, O> && std::indirect_binary_predicate <ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T1*> constexpr replace_copy_result<ranges::borrowed_iterator_t<R>, O> replace_copy( R&& r, O result, const T1& old_value, const T2& new_value, Proj proj = {} ); |  | (since C++20) (until C++26)
template< ranges::input_range R, class O, class Proj = std::identity, class T1 = std::projected_value_t<ranges::iterator_t<R>, Proj>, class T2 = std::iter_value_t<O> > requires std::indirectly_copyable<ranges::iterator_t<R>, O> && std::indirect_binary_predicate <ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T1*> && std::output_iterator<O, const T2&> constexpr replace_copy_result<ranges::borrowed_iterator_t<R>, O> replace_copy( R&& r, O result, const T1& old_value, const T2& new_value, Proj proj = {} ); |  | (since C++26)
| (3) |
template< std::input_iterator I, std::sentinel_for<I> S, class T, std::output_iterator<const T&> O, class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > requires std::indirectly_copyable<I, O> constexpr replace_copy_if_result<I, O> replace_copy_if( I first, S last, O result, Pred pred, const T& new_value, Proj proj = {} ); |  | (since C++20) (until C++26)
template< std::input_iterator I, std::sentinel_for<I> S, class O, class T = std::iter_value_t<O> class Proj = std::identity, std::indirect_unary_predicate<std::projected<I, Proj>> Pred > requires std::indirectly_copyable<I, O> && std::output_iterator<O, const T&> constexpr replace_copy_if_result<I, O> replace_copy_if( I first, S last, O result, Pred pred, const T& new_value, Proj proj = {} ); |  | (since C++26)
| (4) |
template< ranges::input_range R, class T, std::output_iterator<const T&> O, class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred > requires std::indirectly_copyable<ranges::iterator_t<R>, O> constexpr replace_copy_if_result<ranges::borrowed_iterator_t<R>, O> replace_copy_if( R&& r, O result, Pred pred, const T& new_value, Proj proj = {} ); |  | (since C++20) (until C++26)
template< ranges::input_range R, class O, class T = std::iter_value_t<O> class Proj = std::identity, std::indirect_unary_predicate <std::projected<ranges::iterator_t<R>, Proj>> Pred > requires std::indirectly_copyable<ranges::iterator_t<R>, O> && std::output_iterator<O, const T&> constexpr replace_copy_if_result<ranges::borrowed_iterator_t<R>, O> replace_copy_if( R&& r, O result, Pred pred, const T& new_value, Proj proj = {} ); |  | (since C++26)
Helper types |  |
template< class I, class O > using replace_copy_result = ranges::in_out_result<I, O>; | (5) | (since C++20)
template< class I, class O > using replace_copy_if_result = ranges::in_out_result<I, O>; | (6) | (since C++20)

Copies the elements from the source range `[`first`,` last`)` to the destination range beginning at result, replacing all elements satisfying specific criteria with new_value. The behavior is undefined if the source and destination ranges overlap.

1) Replaces all elements that are equal to old_value, using std::invoke(proj, *(first + (i - result))) == old_value to compare.
3) Replaces all elements for which the predicate pred evaluates to true, where the evaluating expression is std::invoke(pred, std::invoke(proj, *(first + (i - result)))).
2,4) Same as (1,3), but uses r as the source range, as if using ranges::begin(r) as first, and ranges::end(r) as last.

The function-like entities described on this page are algorithm function objects (informally known as niebloids), that is:

- Explicit template argument lists cannot be specified when calling any of them.
- None of them are visible to argument-dependent lookup.
- When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

### Parameters

first, last | - | the range of elements to copy
r | - | the range of elements to copy
result | - | the beginning of the destination range
old_value | - | the value of elements to replace
new_value | - | the value to use as a replacement
pred | - | predicate to apply to the projected elements
proj | - | projection to apply to the elements.

### Return value

{last, result + N}, where

1,3) N = ranges::distance(first, last);
2,4) N = ranges::distance(r).

### Complexity

Exactly N applications of the corresponding predicate comp and any projection proj.

### Possible implementation

replace_copy (1,2)
    struct replace_copy_fn
    {
        template<std::input_iterator I, std::sentinel_for<I> S,
                 class O, class Proj = std::identity,
                 class T1 = std::projected_value_t<I, Proj>,
                 class T2 = std::iter_value_t<O>>
        requires std::indirectly_copyable<I, O> &&
                 std::indirect_binary_predicate
                     <ranges::equal_to, std::projected<I, Proj>, const T1*> &&
                 std::output_iterator<O, const T2&>
        constexpr ranges::replace_copy_result<I, O>
            operator()(I first, S last, O result, const T1& old_value,
                       const T2& new_value, Proj proj = {}) const
        {
            for (; first != last; ++first, ++result)
                *result = (std::invoke(proj, *first) == old_value) ? new_value : *first;
            return {std::move(first), std::move(result)};
        }

        template<ranges::input_range R, class O, class Proj = std::identity,
                 class T1 = std::projected_value_t<ranges::iterator_t<R>, Proj>,
                 class T2 = std::iter_value_t<O>>
        requires std::indirectly_copyable<ranges::iterator_t<R>, O> &&
                 std::indirect_binary_predicate
                     <ranges::equal_to,
                      std::projected<ranges::iterator_t<R>, Proj>, const T1*>
        constexpr ranges::replace_copy_result<ranges::borrowed_iterator_t<R>, O>
            operator()(R&& r, O result, const T1& old_value,
                       const T2& new_value, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::move(result),
                           old_value, new_value, std::move(proj));
        }
    };

    inline constexpr replace_copy_fn replace_copy {};
replace_copy_if (3,4)
    struct replace_copy_if_fn
    {
        template<std::input_iterator I, std::sentinel_for<I> S,
                 class O, class T = std::iter_value_t<O>
                 class Proj = std::identity,
                 std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
        requires std::indirectly_copyable<I, O> && std::output_iterator<O, const T&>
        constexpr ranges::replace_copy_if_result<I, O>
            operator()(I first, S last, O result, Pred pred,
                       const T& new_value, Proj proj = {}) const
        {
            for (; first != last; ++first, ++result)
                 *result = std::invoke(pred, std::invoke(proj, *first)) ? new_value : *first;
            return {std::move(first), std::move(result)};
        }

        template<ranges::input_range R, class O, class T = std::iter_value_t<O>
                 class Proj = std::identity,
                 std::indirect_unary_predicate
                     <std::projected<ranges::iterator_t<R>, Proj>> Pred>
        requires std::indirectly_copyable<ranges::iterator_t<R>, O> &&
                 std::output_iterator<O, const T&>
        constexpr ranges::replace_copy_if_result<ranges::borrowed_iterator_t<R>, O>
            operator()(R&& r, O result, Pred pred,
                       const T& new_value, Proj proj = {}) const
        {
            return (*this)(ranges::begin(r), ranges::end(r), std::move(result),
                           std::move(pred), new_value, std::move(proj));
        }
    };

    inline constexpr replace_copy_if_fn replace_copy_if {};

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | list-initialization for algorithms (1-4)

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <complex>
    #include <iostream>
    #include <vector>

    void println(const auto rem, const auto& v)
    {
        for (std::cout << rem << ": "; const auto& e : v)
            std::cout << e << ' ';
        std::cout << '\n';
    }

    int main()
    {
        std::vector<int> o;

        std::array p{1, 6, 1, 6, 1, 6};
        o.resize(p.size());
        println("p", p);
        std::ranges::replace_copy(p, o.begin(), 6, 9);
        println("o", o);

        std::array q{1, 2, 3, 6, 7, 8, 4, 5};
        o.resize(q.size());
        println("q", q);
        std::ranges::replace_copy_if(q, o.begin(), [](int x) { return 5 < x; }, 5);
        println("o", o);

        std::vector<std::complex<short>> r{{1, 3}, {2, 2}, {4, 8}};
        std::vector<std::complex<float>> s(r.size());
        println("r", r);
        #ifdef __cpp_lib_algorithm_default_value_type
            std::ranges::replace_copy(r, s.begin(),
                                      {1, 3}, // T1 gets deduced
                                      {2.2, 4.8}); // T2 gets deduced
        #else
            std::ranges::replace_copy(r, s.begin(),
                                      std::complex<short>{1, 3},
                                      std::complex<float>{2.2, 4.8});
        #endif
        println("s", s);

        std::vector<std::complex<double>> b{{1, 3}, {2, 2}, {4, 8}},
                                          d(b.size());
        println("b", b);
        #ifdef __cpp_lib_algorithm_default_value_type
            std::ranges::replace_copy_if(b, d.begin(),
                [](std::complex<double> z){ return std::abs(z) < 5; },
                {4, 2}); // Possible, since the T is deduced.
        #else
            std::ranges::replace_copy_if(b, d.begin(),
                [](std::complex<double> z){ return std::abs(z) < 5; },
                std::complex<double>{4, 2});
        #endif
        println("d", d);
    }

Output:

    p: 1 6 1 6 1 6
    o: 1 9 1 9 1 9
    q: 1 2 3 6 7 8 4 5
    o: 1 2 3 5 5 5 4 5
    r: (1,3) (2,2) (4,8)
    s: (2.2,4.8) (2,2) (4,8)
    b: (1,3) (2,2) (4,8)
    d: (4,2) (4,2) (4,8)

### See also

ranges::replaceranges::replace_if (C++20)(C++20) | replaces all values satisfying specific criteria with another value (algorithm function object)
replace_copyreplace_copy_if | copies a range, replacing elements satisfying specific criteria with another value (function template)

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